SPECTRA AND STRUCTURE OF SMALL-RING COMPOUNDS .62. RAMAN AND INFRARED-SPECTRA, CONFORMATIONAL STABILITY, VIBRATIONAL ASSIGNMENT OF AB-INITIO CALCULATIONS OF CYCLOPROPYLMETHYLTRIFLUOROSILANE

The Raman (3500-20 cm-1) and infrared (3500-50 cm-1) spectra of gaseou s and solid cyclopropylmethyltrifluorosilane, c-C3H5CH2SiF3, were reco rded. Additionally, the Raman spectrum of the liquid was obtained alon g with qualitative depolarization values. The spectra of the gas and l iquid were interpreted on the basis of the predominance of a conformat ion having a gauche structure and this form exists exclusively in the solid. Spectral evidence to support the existence of a higher energy c is conformer where the SiF3 group is cis to the cyclopropyl ring is ob served in the Raman spectra of the gas and liquid. From studies of the Raman spectra at various temperatures, the conformational enthalpy di fference was determined to be 490 +/- 122 cm-1 (1.4 +/- 0.35 kcal mol- 1) and 548 +/- 110 cm-1 (1.57 +/- 0.32 kcal mol-1) (1 kcal = 4.184 kJ) for the gas and liquid, respectively. The cyclopropyl torsion was obs erved as a weak infrared band centered at 77 cm-1 in the spectrum of t he gas. A complete assignment of the normal vibrational modes is provi ded. Ab initio calculations utilizing the RHF/6-31G and MP2/6-31G* ba sis sets are consistent with the gauche rotamer lying 554 cm-1 (1.6 kc al mol-1) and 296 cm-1 (0.85 kcal mol-1), respectively, lower in energ y than the cis conformation. It is estimated that there is only 4-10% of the higher energy conformer present in the vapor at ambient tempera ture. Optimized structural parameters were obtained for both conformer s with both basis sets and the results should provide good starting pa rameters for the analysis of the electron diffraction data. The force field obtained with the RHF/6-31G basis set was used to perform a nor mal coordinate analysis. These results are compared with corresponding quantities for some similar molecules.